Actuator assembly

Abstract

The acoustic device 10 for generating sound comprises a housing 12 containing a diaphragm 18 coupled to the housing. The diaphragm 18 is driven to generate acoustic waves by an actuator assembly to mo

Description

It has been demonstrated that perceived audio quality can be improved by extending an acoustic device's low frequency extension. That is, increasing the range and quality of low frequency sound reproduction. Improved low frequency extension can also be employed to provide a haptic experience to a user. This can further improve the audio experience of gaming, films and music. Often, acoustic devices have a limited frequency range over which they can reproduce sound of a high quality, for example due to limitations in the actuation mechanisms. Low frequencies often require larger diaphragms which, in turn, require greater forces to move. This can hamper the accurate reproduction of low frequency sounds. It is particularly challenging to manage low frequency reproduction in wearable acoustic devices such as earphones and headphones. This is because earphones and headphones are subject to much greater space and weight restrictions and so it is difficult to incorporate an actuator assembly effective enough to reproduce low frequencies to a high quality, while also being small enough to fit in an earphone or headphone. Summary According to an aspect of the invention, there is provided an acoustic device for generating sound. The acoustic device has a housing, which has a chamber and an opening. The acoustic device further comprises a diaphragm. The diaphragm is arranged within the housing. An actuator assembly is configured to move the diaphragm relative to the housing. The actuator assembly comprises a shape memory alloy, SMA, element. The SMA element is configured to drive movement of the diaphragm. The acoustic device is configured such that movement of the diaphragm generates acoustic waves in the chamber which leave the housing through the opening. The present disclosure provides an acoustic device that overcomes the deficiencies of the prior art. In particular, the acoustic device of the present disclosure is compact but can provide a large driving force, thus providing increased freedom for the design of acoustic devices such as those used in headphones and earphones. The large driving force allows larger diaphragms to be used in smaller spaces, improving low frequency sound reproduction and haptic effects. The acoustic device may be configured for use within a wearable audio system such as an earphone or headphone. At least part of the housing may have outer diameters to fit within a user's ear. The housing may, for example, be cylindrical. A shape memory alloy (SMA) may describe an alloy that is deformable when cold, but returns to a pre-deformed or remembered shape when heated. Heating may be achieved by running a current through the SMA element, for example. A SMA element may therefore be used as an actuator - for example by running a current through a deformed SMA element, causing it to increase in temperature and return to its pre-deformed shape - thus exerting a force on a connected component. The SMA element as described herein may have associated circuitry in order to selectively activate the element (e.g. by running a current through the element and heating the SMA element). The associated circuitry may form part of the acoustic device, for example as part of the actuator assembly, however will not be discussed in detail herein. The use of an SMA element may allow the actuator assembly to be more compact - i.e. take up less room inside an earphone - than existing systems. The SMA element may be a wire. An actuator assembly employing a wire as its primary actuator, which may be more space efficient than using coils and / or magnets such as for balanced armature or dynamic drivers. The SMA element may be substantially parallel to the diaphragm. A parallel arrangement allows the actuator assembly to stay within the same footprint of the diaphragm while minimising the thickness of the device as a whole - again saving space. The SMA element and / or diaphragm may be located within the chamber of the housing. The chamber may be in communication with the opening. Alternatively, the SMA element may be at a non-zero acute angle to the diaphragm. The angle between the diaphragm and the SMA element may vary during use, as the diaphragm moves. It is to be understood that discussion herein relating to relative angles is used to describe the system in a neutral state with the diaphragm unloaded and in its rest (e.g. undeflected) position. The relative angle between the SMA element and the diaphragm may adjust the gearing of the actuator assembly - that is, the distance the diaphragm is driven compared to the extension or contraction of the SMA element. Broadly speaking, the more acute the angle between the SMA element and the diaphragm, the higher the degree of gearing - i.e. the more the diaphragm is moved for every unit of contraction of the SMA element. Defining a specific SMA element angle within the actuator assembly can therefore control the amount of movement of the diaphragm to produce a certain audio profile. The actuator assembly may comprise a linkage. The SMA element may be connected to the diaphragm via the linkage. A linkage may be configured to convert movement in a first direction (e.g. parallel to the axis of the SMA element) to a movement in a direction at an angle to the first movement (e.g. perpendicular to the plane of the diaphragm). The linkage may be a connecting rod. The connecting rod may be arranged to apply a transverse driving force to the diaphragm. The transverse driving force may be in a direction perpendicular to the plane of the diaphragm. The connecting rod may be pivotally connected to the SMA element and / or diaphragm. The connecting rod may be rigid. Alternatively, the connecting rod may be flexible. In some examples, a part of the connecting rod may be rigid and a part of the connecting rod may be flexible. The flexible section of the connecting rod may be located at an end of the connecting rod - for example where it connects to the housing or SMA element, to allow the SMA rod to pivot about that connection. An axis of the connecting rod and a plane of the diaphragm may define an acute, non-zero angle. The angle between the axis of the connecting rod and the plane of the diaphragm may be less than 60 degrees. Corresponding comments apply to the linkage. The gearing - i.e. the amplification of the extension of the SMA element compared to the movement of the diaphragm may be dependent on the angle between the axis of the connecting rod and plane of the diaphragm. Broadly speaking, the smaller the angle, the higher the amount of movement amplification (and corresponding force reduction). The angle may therefore be selected to provide a desired diaphragm movement profile. The angle between the axis of the connecting rod and the plane of the diaphragm may be non-zero and less than any one of 90, 80, 70, 60,50,40, 30, 20 or 10 degrees. Alternatively, the angle may be between any two of the above-listed numbers. An axis of the connecting rod and an axis of the SMA element may define an acute, non-zero angle. The angle between the axis of the connecting rod and the axis of the SMA element may be less than 60 degrees. Corresponding comments apply to the linkage. The gearing - i.e. the amplification of the extension of the SMA element compared to the movement of the diaphragm may be dependent on the angle between the axis of the connecting rod and axis of the SMA element. Broadly speaking, the smaller the angle, the higher the amount of movement amplification (and corresponding force reduction). The angle may therefore be selected to provide a desired diaphragm movement profile. The angle between the axis of the connecting rod and the axis of the SMA element may be non-zero and less than any one of 90, 80, 70, 60,50,40, 30, 20 or 10 degrees. Alternatively, the angle may be between any two of the above-listed numbers. The most advantageous angle between the connecting rod ( / linkage) and the diaphragm and / or SMA element will depend on the specific gearing required which, in turn, will depend on the specific application of the device. The basic principles of the acoustic device described herein can be readily modified and adapted to work in a wide range of applications - for example by adapting the relative angles of the linkage and associated components. The actuator may comprise a second linkage. The second linkage may be configured to anchor the SMA element with respect to the housing. The second linkage may be a connecting rod. The second linkage (e.g. the ends thereof) may be connected to the SMA element and the housing. The second linkage may increase the driving movement of the diaphragm for a set contraction of the SMA element (e.g. compared to an arrangement without a second linkage). The second linkage may cause the SMA element to deflect towards the anchoring point of the second linkage (e.g. on the housing), thus causing the diaphragm to move further in that direction than if the SMA element were in its neutral position. The actuator assembly may comprise at least one pair of opposing SMA elements. The SMA elements may be provided in opposing pairs such that the actuator assembly is configured to drive the diaphragm in two opposing directions. An SMA element may only be configured to exert a driving force in a single direction (e.g. the direction of contraction of an SMA wire). Therefore, in order to drive the diaphragm in more than one direction (e.g. opposing directions) a pair of SMA elements may be used. Opposing the SMA elements allows the diaphragm to be driven in a first direction, and then returned in a second, opposing, direction. Where an opposing pair of SMA elements is used, the associated circuitry may be configured to selectively make one of the elements the "hot" element and (while the other element is left as the "cold" element). The "cold" element may expand as the "hot" element contracts. This may drive the diaphragm in a first direction. The "hot" and "cold" elements may then switch, to drive the diaphragm in a second direction. The opposing pair of SMA elements may be connected in series for driving movement of the diaphragm in opposing directions. The connecting rod may be connected at the interface of the opposing SMA elements. All connections for the SMA elements (e.g. to the housing, a second SMA element, diaphragm, linkage, connecting rod, etc...) may be achieved using crimps. In one specific arrangement, a first SMA element may be connected to the housing at one end and to a first end of a second SMA element at the other end. The second SMA element may be connected to the housing at its second end. That is, the SMA elements may be connected in series, across the housing - e.g. the chamber of the housing. In this arrangement, the interface of the first and second SMA elements moves in two opposing directions depending on which of the SMA elements is activated (i.e. becoming the "hot" element), and which is the cold element. A linkage (e.g. connecting rod) may be connected to the interface of the first and second SMA elements, allowing the diaphragm to be moved in two opposing directions accordingly. The linkage may be a lever arm configured to rotate under the action of the SMA element. The actuator assembly may be configured to rotate under the action of the SMA element. The lever arm may be configured to convert its rotation to a lateral movement (e.g. through the use of a lever arm), or to apply a moment to an end of the diaphragm to cause bending displacement of the diaphragm. The actuator assembly may comprise at least one pair of opposing SMA elements for driving movement of the diaphragm in opposing directions. The opposing SMA elements may be connected to opposing sides of the lever arm. Alternatively, the opposing SMA elements may be connected to opposing lever arms on opposing sides of the diaphragm. A portion of the diaphragm may have a reduced thickness, to allow the rest of the diaphragm to deflect, e.g. pivot, about this location. The SMA element may be directly connected to the diaphragm. Where it is stated that the SMA element may be directly connected to the diaphragm, it is to be understood that "directly" refers to a connection where the driving force and movement of the SMA element is not changed before being applied to the diaphragm - e.g. there is no linkage or lever arm to change the direction or quantum of force. The term "directly" as used in this way does not require the SMA element to be connected to the diaphragm without intermediary. It is understood that there may be additional components (e.g. crimps, clamps, buffers, pads,...) to allow the SMA element to be mechanically coupled to the diaphragm. Where it is stated that the SMA element is connected to a further component, it may be an end of the SMA element that is connected. The SMA element may be arranged substantially perpendicular to a plane of the diaphragm. Alternatively, the SMA element and a plane of the diaphragm may define an acute, non-zero, angle. The angle may be non-zero and less than any one of 90, 80, 70,60,50,40, 30, 20 or 10 degrees. Alternatively, the angle may be between any two of the above-listed numbers. As discussed above, the angle between the SMA element and the diaphragm determines the degree of gearing applied to the movement of the diaphragm. The smaller the angle, the larger the degree of movement amplification (and force reduction). The angle can therefore be selected to match the application of the acoustic device. Low frequency sound often benefits from a larger diaphragm, which requires a larger driving force. Acoustic devices for low-frequency sound reproduction may therefore use an angle between the diaphragm and the SMA element of 90 degrees or close to 90 degrees (e.g. over 70 or 80 degrees). Using a smaller angle (e.g. less than 50, 40 or 30 degrees) may increase diaphragm displacement, but reduce the force applied to the diaphragm and so may be best suited to smaller diaphragms. The actuator assembly may comprise at least one pair of opposing SMA elements, the opposing SMA elements being configured to drive the diaphragm in opposing directions, wherein each SMA element is directly connected to the diaphragm and the opposing SMA elements are located on opposing sides of the diaphragm. The acoustic device may further comprise a membrane connecting the diaphragm to the housing, wherein the membrane is configured to expand and contract to facilitate the movement of the diaphragm. The membrane may be made of an elastic material (e.g. more elastic, with a lower Young's Modulus) than the diaphragm. It may be desirable for the diaphragm to stay as flat as possible during use. A membrane may therefore be employed to deform as the diaphragm moves, with the diaphragm staying substantially planar. The membrane may be located around the periphery, or part of the periphery, of the diaphragm. The actuator assembly may further comprise a biasing member arranged to oppose the driving action of the SMA element. In an example where only a single SMA element is provided, a driving force is only applied by the SMA element in a single direction. A returning force may be provided by a biasing member. An example biasing member may be a spring, which may be arranged to oppose the driving action of the SMA element. In some examples, the biasing member may be the inherent elasticity of the diaphragm and / or membrane. The diaphragm may be a cantilever, wherein one end or side of the diaphragm is fixed with respect to the housing and a second end or side of the diaphragm is moveable with respect to the housing. This may be a robust and compact diaphragm arrangement. The acoustic device may further comprise a second diaphragm arranged within the housing. The second diaphragm may be configured to move within the housing. The use of two diaphragms within an actuator assembly may improve low-frequency extension. The SMA element may be configured to drive movement of the second diaphragm. The diaphragm and second diaphragm may be substantially symmetric about a plane through a centre axis of the housing. The diaphragms may be substantially parallel to each other. The diaphragms may be substantially parallel to the SMA element. Alternatively, the SMA element may be at a non-zero acute angle to each of the diaphragms. The actuator assembly may be arranged between the diaphragms and may be configured such that, as the SMA element contracts, the diaphragms are moved towards each other. The housing may comprise a vent for venting pressurised air. The vent may be arranged on the opposite side of the diaphragm to the chamber and / or the opening. The vent may be configured to allow pressurised air to exit the housing and avoid the pressure affecting audio performance. The housing may have a diameter of less than 10mm. The diameter may be perpendicular to the plane of the diaphragm, and / or axis of the SMA element. The housing may be configured to fit within a wearable audio system - such as an earphone or headphone. According to another aspect of the present invention, there is a headphone comprising the acoustic device of any preceding claim. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of a first acoustic device; Figure 2 is a schematic view of a second acoustic device; Figure 3 is a schematic view of a third acoustic device; Figure 4 is a schematic view of a fourth acoustic device; Figure 5 is a schematic view of a fifth acoustic device; and Figure 6 is a schematic view of a sixth acoustic device. Detailed description Figure 1 is a schematic cross-section of an acoustic device 10. The acoustic device 10 includes a housing 12, which has a chamber 14 and an opening 16. The opening 16 is in communication with the chamber 14. The acoustic device 10 further includes a diaphragm 18 arranged within the housing 12 and an actuator assembly comprising a SMA element 20. During use, the SMA element 20 drives movement of the diaphragm 18, within the housing 12. This produces acoustic waves in the chamber 14 which leave the acoustic device 10 via the opening 16. The housing 12 may be of substantially any shape, provided it is suitable for use as part of an acoustic device 10 and for housing the other features described herein. The shape and scale of the housing 12 may depend on the specific use of the acoustic device. In a loudspeaker, the housing 12 (and acoustic device 10 overall) may be larger than one for use in a headphone or earphone. The housing 12 may have a diameter - e.g. a largest diameter or the diameter of a cylindrical section of the housing - less than 15mm, 10mm, or even 8mm. The chamber 14 is configured for the production of acoustic waves for sound in cooperation with the diaphragm 18. The chamber 14 of the present example is cylindrical, but may of any other state suitable for use in the production of acoustic waves. In the acoustic device 10 of Figure 1, the opening 16 is a circular opening on one side of the chamber 14, configured such that acoustic waves can leave the chamber 14 and enter a listener's ear. The housing 12 also comprises internal supports arranged to locate and fix other parts of the acoustic device 10 within the chamber 14. For example, in the example of Figure 1, the housing 12 comprises a plurality of flanges for use in securing the diaphragm 18 with respect to housing 12. The diaphragm 18 is arranged in the chamber 14 of the housing 12. In some examples, the diaphragm 18 may be arranged adjacent or contiguous with the chamber 14. The diaphragm 18 is configured to move within the housing 12 (and more specifically the chamber 14 thereof). Movement of the diaphragm 18 causes movement of a fluid - e.g. air - within the chamber 14. This, in turn, creates pressure variations, or acoustic waves, which leave the housing 12 via the opening 16 and allow a listener to hear the reproduced sound. In the example of Figure 1, the diaphragm is fixed directly to a flange 24 of the housing 12 on one side (right hand side of Figure 1). In this particular example, the diaphragm 18 is shown as being rigidly fixed with respect to the housing 12. The diaphragm may comprise a thinned, weakened or scored section adjacent the housing connection to facilitate bending at that point. This may allow the connection to behave more like a pivot connection. Alternatively, the diaphragm 18 may be connected to the housing via a hinge or pivot joint. The diaphragm 18 being configured to pivot about its connection to the housing 12 reduces the amount the diaphragm 18 bends during use. Minimising bending of the diaphragm 18 can improve sound quality. On the opposing side of the diaphragm 18 (the left side in Figure 1), the diaphragm 18 is connected to the housing 12 by a membrane 26. The membrane 26 may be configured to allow movement of the diaphragm 18. The membrane 26 may also be configured to minimise bending of the diaphragm 18. The membrane 26 may be an elastic material. The membrane 26 may have a lower Young's Modulus than the diaphragm 18. The diaphragm 18 is arranged with its planar surface adjacent and facing the chamber 14 such that perpendicular movement of the diaphragm 18 moves a large volume of air within the chamber. The diaphragm 18 and chamber 14 may be of any suitable shape for producing acoustic waves. For example, the diaphragm may be circular or rectangular (incl. square). If a rectangle, the diaphragm may have rounded corners. The SMA element 20 forms part of an actuator assembly and is configured to drive movement of the diaphragm 18. The diaphragm 18 is driven in a direction perpendicular to the plane of the diaphragm 18. The specific configuration of the actuator assembly may vary within the present disclosure, but an SMA element 20 is used to drive movement of the diaphragm 18. In the example of Figure 1, the SMA element 20 is arranged substantially perpendicular to the diaphragm 18. For example, the SMA element 20 may define an angle of between 80 and 90 degrees, or 85 and 90 degrees, with the diaphragm 18. The SMA element 20 is connected to the housing 12 and the diaphragm 18. In the present example, the SMA element 20 is connected using crimps 30. The SMA element 20 is also connected to circuitry (not shown) configured to selectively heat the SMA element 20. An SMA element 20 can only exert a force in a single direction. As such, in order to fully control the displacement and return of the diaphragm 18, a returning force is required. In the example of Figure 1, a biasing means (here, a spring 22) is arranged opposing the SMA element 20. The spring 22 is arranged on the other side of the diaphragm 18 and is arranged to oppose the SMA element force applied to the diaphragm 18 and return the diaphragm 18 towards the neutral position when the SMA element 20 stops driving the diaphragm 18. As such, reciprocal movement of the diaphragm 18 can be achieved by alternating between activating the SMA element 20 to cause it to contract and drive the diaphragm 18 in a first direction (downwards, in Figure 1), and then allowing the SMA element 20 to cool and expand, such that the spring 22 can urge the diaphragm 18 back in a second direction (upwards in Figure 1). In this manner, acoustic waves can be produced in the chamber 14. In other examples, not shown, the spring 22 may be omitted and the restoring force may instead be provided by inherent elasticity of the system -e.g. the diaphragm 18, membrane 26 and / or supporting members of the housing 12. Turning now to Figure 2, a similar arrangement is shown. The majority of comments made above with respect to Figure 1 apply here, mutatis mutandis. Like reference numerals are used for corresponding features. Only the differences between the examples of Figure 1 and 2 will be discussed in detail. In Figure 2, the spring 22 of Figure 1 is replaced with an opposing SMA element 21. This second SMA element 21 is arranged on the opposing side of the diaphragm 18 and is configured to selectively provide a force in an opposing direction to that of the first SMA element 20. That is, the first 20 and second 21 SMA elements are connected such that at a first time the first SMA element 20 is "hot" and drives the diaphragm 18 as the second SMA element 21 is "cold" and is free to deform. The diaphragm 18 therefore moves towards the first SMA element 20. In a second time period, the second SMA element 21 becomes "hot" and drives the diaphragm 18 while the first SMA element 20 is "cold" and is free to deform. The diaphragm 18 therefore moves towards the second SMA element, back to the neutral position and beyond in the direction of the second SMA element. In this way, the movement of the diaphragm 18 can be controlled to produce an acoustic wave. Turning now to Figure 3, the acoustic device is similar to that of Figure 2, however the first SMA element 20 and second SMA elements 21 have a different arrangement. In Figure 3, both of the first 20 and second 21 SMA elements are arranged at an oblique angle to the diaphragm 18. Specifically, the first SMA element 20 and second SMA element 21 define an angle a with the diaphragm 18 that is a non-zero acute angle. It is noted that, in Figure 3, the diaphragm 18 is shown deflected slightly towards the first SMA element 20 and so the two SMA elements 20,21 do not define the same angle with the diaphragm 18. It is also noted that as the diaphragm 18 moves, the angle will vary. For the sake of the present disclosure, where angles between an SMA element 20, 21 and the diaphragm 18 are discussed, they refer to an angle with the diaphragm 18 in the neutral position. Arranging the SMA elements 20, 21 at a non-zero acute angle with respect to the diaphragm 18 allows the acoustic device 10 to benefit from a movement amplification or "gearing" effect, whereby the distance moved by the diaphragm is greater than the degree of contraction of the SMA element 20, 21. This comes at the expense of applied force. Having the SMA elements 20, 21 arranged perpendicular to the diaphragm 18 as shown in Figure 2 maximises the amount of force that can be applied to the diaphragm 18. This can be beneficial as low frequency acoustic waves require large diaphragms, which require greater driving forces. Having the SMA elements 20, 21 arranged at a non-zero acute angle with respect to the diaphragm 18 amplifies the movement of the diaphragm, but reduces the driving force. The more acute the angle, the more the movement is amplified, but the force decreased. In Figure 4, an alternative actuator assembly is shown. The example of Figure 4 includes a first and second SMA element 20, 21 arranged on the same side, and parallel to, the diaphragm 18. The first SMA element 20 and second SMA element 21 are arranged end to end, with the distal ends of each SMA element 20, 21 connected to the housing 12. This arrangement may be a particularly compact arrangement. The actuator assembly comprises linkages in the form of first connecting rod 32 and second connecting rod 33. The first connecting rod 32 connects the interface of the first SMA element 20 and second SMA element 21 to the diaphragm 18. The second connecting rod 33 connects and anchors the SMA elements 20, 21 to the housing 12. Second connecting rod 33 also increases the driving movement applied to the diaphragm 18 by a certain contraction distance of the respective SMA elements 20, 21. The first connecting rod 32 and second connecting rod 33 are angled with respect to the diaphragm 18 (a first angle - angle P) and the SMA elements 20, 21 (a second angle - angle y). In the example of Figure 4, where the SMA elements 20, 21 are substantially parallel to the diaphragm 18, these angles are substantially equal, however they may not be in all acoustic devices. As before, the use of opposing SMA elements 20, 21 allows the diaphragm 18 to be actively driven in opposing directions. As the SMA elements 20, 21 expand and contract, the mid-point connection with the first connecting rod 32 moves parallel to the diaphragm 18. This parallel force is converted, by the first connecting rod 32 into a perpendicular driving force for the diaphragm 18. The degree of "gearing" achieved - i.e. the degree to which the force and movement of the SMA elements 20, 21 are amplified / reduced is dependent on both of the first and second angles P and y. The more acute the angles first and second angles P and y are, the greater the degree of movement amplification and force reduction. Figure 5 is a schematic of a further example. In the acoustic device 10 of Figure 5, a second diaphragm 19 is introduced. The second diaphragm 19 is arranged on an opposing side of the actuating assembly. The second diaphragm 19 is substantially symmetric to the first diaphragm 18, about axis of the SMA elements 20, 21. In the example of Figure 5, rather than being connected to the housing 12, the second connecting rod 33 is connected to the second diaphragm 19. The inclusion of a second diaphragm 19 may change the amount of air which can be moved within the chamber 14 and thus the nature of the acoustic wave produced. In this arrangement, the inclusion of the second diaphragm 19 increases the surface area of air being moved by the diaphragms 18, 19, but reduces the distance each diaphragm 18, 19 moves for a given SMA contraction length. The inclusion of an additional diaphragm 18,19 may improve the production of low frequency sounds. The acoustic device 12 of Figure 5 also includes a vent 34. In this example, the vent 34 is arranged on an opposing side of the housing 12 to the opening 16. The vent 34 allows pressurized air to escape from the housing 12. Figure 6 relates to an alternative actuator assembly. In the acoustic device 12 of Figure 6, the first and second SMA elements 20,21 are arranged on opposing sides of the diaphragm 18. Both of the SMA elements 20, 21 are parallel to the diaphragm 18. The opposing SMA elements 20, 21 are connected to opposing sides of a lever arm 36. The SMA elements 20, 21 and lever arm 36 are configured to convert a movement parallel to the plane of the diaphragm 18 to a perpendicular movement. At such, the each SMA element 20,21 is connected to a lever arm 36 mounted on a pivot 38. The diaphragm 18 is also attached to a lever arm 40 which is arranged perpendicular to the SMA element lever arm 36. The above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. The present invention has been described above purely by way of example. Modifications in detail may be made to the present invention within the scope of the claims as appended hereto. Furthermore, features from one example may be combined with an alternative example unless such a combination is explicitly precluded. For example, the acoustic devices described herein may be modified such that the diaphragm is not necessarily disposed within a chamber of a housing and / or the chamber may not comprise an opening. In particular, the following is disclosed: 1. An acoustic device for generating sound, comprising: a housing; a diaphragm coupled to the housing; an actuator assembly configured to move the diaphragm relative to the housing; wherein the actuator assembly comprises a shape memory alloy, SMA, element configured to drive movement of the diaphragm; wherein the acoustic device is configured such that movement of the diaphragm generates acoustic waves. 2. The acoustic device of item 1, wherein the SMA element is substantially parallel to the diaphragm. 3. The acoustic device of item 1 or item 2, wherein the actuator assembly comprises a linkage and the SMA element is connected to the diaphragm via the linkage. 4. The acoustic device of item 3, wherein the linkage is a connecting rod arranged to apply a transverse driving force to the diaphragm. 5. The acoustic device of item 4, wherein an axis of the connecting rod and a plane of the diaphragm define an acute, non-zero angle. 6. The acoustic device of item 5, wherein the angle between the axis of the connecting rod and the plane of the diaphragm is less than 45 degrees. 7. The acoustic device of any of items 4 to 6, wherein an axis of the connecting rod and an axis of the SMA element define an acute, non-zero angle. 8. The acoustic device of item 7, wherein the angle between the axis of the connecting rod and the axis of the SMA element is less than 45 degrees. 9. The acoustic device of any of items 3 to 8, wherein the actuator comprises a second linkage, wherein the second linkage anchors the SMA element with respect to the housing. 10. The acoustic device of any of items 4 to 9, wherein the actuator assembly comprises at least one pair of opposing SMA elements connected in series for driving movement of the diaphragm in opposing directions, optionally wherein the connecting rod is connected at the interface of the opposing SMA elements. 11. The acoustic device of item 3, wherein the linkage is a lever arm configured to rotate under the action of the SMA element. 12. The acoustic device of item 11, wherein the actuator assembly comprises at least one pair of opposing SMA elements for driving movement of the diaphragm in opposing directions, wherein the opposing SMA elements are connected to: opposing sides of the lever arm; or opposing lever arms on opposing sides of the diaphragm. 13. The acoustic device of item 1, wherein the SMA element is directly connected to the diaphragm. 14. The acoustic device of item 13, wherein the SMA element is substantially perpendicular to a plane of the diaphragm. 15. The acoustic device of item 13, wherein the SMA element and a plane of the diaphragm define an acute, non-zero, angle. 16. The acoustic device of any of items 13 to 15, wherein the actuator assembly comprises at least one pair of opposing SMA elements, the opposing SMA elements being configured to drive the diaphragm in opposing directions, wherein each SMA element is directly connected to the diaphragm and the opposing SMA elements are located on opposing sides of the diaphragm. 17. The acoustic device of any preceding item, further comprising a membrane connecting the diaphragm to the housing, wherein the membrane is configured to expand and contract to facilitate the movement of the diaphragm. 18. The acoustic device of any preceding item, wherein the actuator assembly further comprises a biasing member arranged to oppose the driving action of the SMA element. 19. The acoustic device of any preceding item, wherein the diaphragm is a cantilever, wherein one end or side of the diaphragm is fixed with respect to the housing and a second end or side of the diaphragm is moveable with respect to the housing. 20. The acoustic device of any preceding item, further comprising a second diaphragm coupled to the housing and configured to move relative to the housing. 21. The acoustic device of item 20, wherein the SMA element is configured to drive movement of the second diaphragm. 22. The acoustic device of item 20 or item 21, wherein the diaphragm and second diaphragm are substantially symmetric about a plane through an axis of the SMA element. 23. The acoustic device of any preceding item, wherein the housing comprises a vent for venting pressurised air. 24. The acoustic device of any preceding item, wherein the housing has a diameter of less than 10mm. 5 25. An audio headphone comprising the acoustic device of any preceding item.

Claims

1. An acoustic device for generating sound, comprising:a housing having a chamber and an opening;a diaphragm arranged within the housing;an actuator assembly configured to move the diaphragm relative to the housing;wherein the actuator assembly comprises a shape memory alloy, SMA, element configured to drive movement of the diaphragm;wherein the acoustic device is configured such that movement of the diaphragm generates acoustic waves in the chamber which leave the housing through the opening.

2. The acoustic device of claim 1, wherein the SMA element is substantially parallel to the diaphragm.

3. The acoustic device of claim 1 or claim 2, wherein the actuator assembly comprises a linkage and the SMA element is connected to the diaphragm via the linkage.

4. The acoustic device of claim 3, wherein the linkage is a connecting rod arranged to apply a transverse driving force to the diaphragm.

5. The acoustic device of claim 4, wherein an axis of the connecting rod and a plane of the diaphragm define an acute, non-zero angle.

6. The acoustic device of claim 5, wherein the angle between the axis of the connecting rod and the plane of the diaphragm is less than 45 degrees.

7. The acoustic device of any of claims 4 to 6, wherein an axis of the connecting rod and an axis of the SMA element define an acute, non-zero angle.

8. The acoustic device of claim 7, wherein the angle between the axis of the connecting rod and the axis of the SMA element is less than 45 degrees.

9. The acoustic device of any of claims 3 to 8, wherein the actuator comprises a second linkage, wherein the second linkage anchors the SMA element with respect to the housing.

10. The acoustic device of any of claims 4 to 9, wherein the actuator assembly comprises at least one pair of opposing SMA elements connected in series for driving movement of the diaphragm in opposing directions.

11. The acoustic device of claim 3, wherein the linkage is a lever arm configured to rotate under the action of the SMA element.

12. The acoustic device of claim 11, wherein the actuator assembly comprises at least one pair of opposing SMA elements for driving movement of the diaphragm in opposing directions, wherein the opposing SMA elements are connected to:opposing sides of the lever arm; oropposing lever arms on opposing sides of the diaphragm.

13. The acoustic device of claim 1, wherein the SMA element is directly connected to the diaphragm.

14. The acoustic device of claim 13, wherein the SMA element is substantially perpendicular to a plane of the diaphragm.

15. The acoustic device of claim 13, wherein the SMA element and a plane of the diaphragm define an acute, non-zero, angle.

16. The acoustic device of any of claims 13 to 15, wherein the actuator assembly comprises at least one pair of opposing SMA elements, the opposing SMA elements being configured to drive the diaphragm in opposing directions, wherein each SMA element is directly connected to the diaphragm and the opposing SMA elements are located on opposing sides of the diaphragm.

17. The acoustic device of any preceding claim, further comprising a membrane connecting the diaphragm to the housing, wherein the membrane is configured to expand and contract to facilitate the movement of the diaphragm.

18. The acoustic device of any preceding claim, wherein the actuator assembly further comprises a biasing member arranged to oppose the driving action of the SMA element.

19. The acoustic device of any preceding claim, wherein the diaphragm is a cantilever, wherein one end or side of the diaphragm is fixed with respect to the housing and a second end or side of the diaphragm is moveable with respect to the housing.

20. The acoustic device of any preceding claim, further comprising a second diaphragm arranged within the housing and configured to move within the housing.

21. The acoustic device of claim 20, wherein the SMA element is configured to drive movement of the second diaphragm.

22. The acoustic device of claim 20 or claim 21, wherein the diaphragm and second diaphragm are substantially symmetric about a plane through an axis of the SMA element.

23. The acoustic device of any preceding claim, wherein the housing comprises a vent for venting pressurised air.

24. The acoustic device of any preceding claim, wherein the housing has a diameter of less than 10mm.

25. An audio headphone comprising the acoustic device of any preceding claim.

Images